This invention relates generally to devices for cooking food type items such as french fries, chicken, fish and the like in an industrial fryer.
Industrial fryers are typically constructed with a tank that holds oil and a device that generates hot gas that runs through individually fired, densely packed pipes built into the bottom of the tank. The pipes become hot causing the oil to be heated to a high temperature. Food then is placed on a conveyor and transported through the oil. When a batch of cold food to be cooked is immersed in the cooking oil, a substantial temperature drop generally occurs. This temperature drop is highly undesirable.
With an industrial fryer, it is desirable to have controlled longitudinal temperature profiles within the fryer so that the food may be cooked at different temperatures optimized for the particular product. By longitudinal profile, it is meant that the temperature of oil at the different locations along the path of food flow has a defined characteristic. When one large volume of product is passing through and thus receiving a large amount of heat, the oil in that region would tend to get cooler unless increased heat were input to the oil in that region. Further, with a fixed gas pipe arrangement, there is a predefined limit as to how heat can be input in a region. For example, the apparent way to increase the input heat (i.e. number of BTU's transferred) would be to increase the temperature of the pipes by firing the burners hotter. However, there is a practical limit such as, for example, 450.degree. or 500.degree. F. above which the pipes cannot be raised without degrading the oil at an unsatisfactory rate. For example, if the pipes were maintained at a temperature of approximately 500.degree. F., the oil contacting the surface of the pipes would degrade at a rate which would require the oil to be changed relatively frequently. As a result, to obtain a desired longitudinal profile, certain regions or zones are reduced in temperature rather than increasing a relatively higher temperature zone above a practical limit.
Another drawback of this well-known device is that as heat flow is derived from free convection, dead spots, where the food is at an unpredictable temperature may occur throughout the oil. A third drawback of the hot gas fryer is that as the pipes located on the bottom of the pan are heated to high temperature, charring or soot may occur on the pipes. This charring or soot causes the fryer to operate at a lower efficiency.
U.S. Pat. No. 4,623,544 to Highnote discloses a batch type fryer/cooker assembly for cooking foods at a predetermined constant temperature. The oil is removed from the cooking chamber, filtered, heated, and then turbulently recycled back to the cooking chamber. However, because the system is a batch system, the food throughout is severely restricted. Also, because the food is stationary, it is impractical to obtain a changing temperature profile during the cooking of a particular food product.